NA
NA
The present invention relates to aldehyde resins having low amounts of free aldehydes. More particularly the present invention relates to aldehyde resins formed in the presence of an amino acid. In the invention aldehyde resins refer to resins derived from the reactions of a phenol, urea, melamine or a mixture thereof and an aldehyde. Examples of aldehyde resins include phenol formaldehyde resins, urea formaldehyde resins, melamine formaldehyde resins, melamine-urea-formaldehyde resins, and the like. These resins are well known in the art.
Phenol formaldehyde resins were the first true synthetic resins to gain commercial acceptance early in the twentieth century. These phenolic resins are the product of the reaction between phenol and formaldehyde. Novalacs are acid catalyzed phenol formaldehyde resins where typically an excess of phenol used. Resoles are the base catalyzed reaction product of phenol and an excess of formaldehyde. In commercial production resoles are normally processed to a workable viscosity; then subsequently polymerized to high molecular weight polymers by simple heating. Urea formaldehyde resins are typically prepared by the condensation of urea and formaldehyde at a pH of between 4 and 7 and at a temperature close to boiling point. Melamine formaldehyde and melamine-urea-formaldehyde resins undergo condensation reactions with an aldehyde in a manner analogous to that of urea. U.S. Pat. No. 5,681,917 discloses a method of making melamine-urea-formaldehyde resins and is herein incorporated by reference.
A problem that exists with aldehyde resin systems is the amount of free formaldehyde that exists in the resins both during storage and upon cure. Formaldehyde is considered toxic and a carcinogen. The American Conference of Governmental and Industrial Hygienists has lowered its TLV to 0.3 ppm. Due to these health concerns much effort has been expended attempting to obtain aldehyde resins with reduced free formaldehyde levels.
An abstract of Japanese patent application 60149638 discloses the use of polyvinyl alcohol to reduce the odor from free formaldehyde in foams produced from resole type phenol-formaldehyde resins. U.S. Pat. No. 3,917,558 discloses the use of nitro compounds such as nitromethane to reduce the concentration of free formaldehyde in phenol-formaldehyde resins. U.S. Pat. No. 5,705,537 discloses the addition of a proteinaceous material, cysteine, glutamic acid, glycine, isoleucine, lysine, phenylalanine, serine tryptophan or mixtures thereof to a phenolic foam composition consisting of a phenol formaldehyde resole resin. The reference discloses the addition of the aldehyde reducing agent to the already formed resin. The use of melamine, urea and sodium sulfite have also been suggested for use as scavengers for formaldehydes. Some reduction in free formaldehyde concentration was noted in uncured resins where these scavengers were used, however during curing at high temperatures free formaldehyde levels increased over precure levels.
There are no suggestions in the art to utilize amino acids and in particular glycine to reduce the free formaldehyde in aldehyde resins by adding the amino acids to the reaction mixture of the aldehyde resin.
The present invention describes aldehyde resins having reduced free formaldehyde. Particularly the invention relates to the use of amino acids to effectively reduce the amount of free aldehyde in the resins. More particularly the invention relates to the use of glycine as a component in aldehyde resins to reduce the free formaldehyde in aldehyde resins. The use of glycine in aqueous aldehyde resin systems also provides the added benefit of increased water tolerance over time.
NA
The present invention describes aldehyde resins having reduced amounts of free formaldehyde and methods of making the resins. In addition when glycine is added to the reaction mixture of an aqueous aldehyde resin system the resin especially resoles exhibit increased water tolerance over time.
The aldehyde resins for which amino acids will function to reduce free aldehyde include those aldehyde resins known in the art such as phenol formaldehyde, urea formaldehyde, melamine formaldehyde or melamine-urea-formaldehyde.
Phenols used in the preparation of phenol formaldehyde resins include one or more of the phenols which have heretofore been employed in the formation of phenolic resins and are not substituted at either the two ortho positions or at one ortho position and the para position. Such unsubstituted positions are necessary for the polymerization reaction.
Any one or all of the remaining carbon atoms of the phenol ring can be substituted. The nature of the substituent can vary widely and it is only necessary that the substituent not interfere with the polymerization of the aldehyde with the phenol at the ortho and/or para position. Substituted phenols employed in the formation of phenolic resins include alkyl substituted phenols, aryl substituted phenols, cyclo-alkyl substituted phenols, aryloxy substituted phenols, and halogen substituted phenols. The foregoing substituents can contain from 1 to 26 carbon atoms and preferably from 1 to 12 carbon atoms.
Specific examples of suitable phenols include 2,6-xylenol, o-cresol, p-cresol, 3,5-xylenol, 2,3,4-trimethyl phenol, 3-ethyl phenol, 3,5-diethyl phenol, p-butyl phenol, 3,5-dibutyl phenol, p-amyl phenol, p-cyclohexyl phenol, p-octyl phenol, 3,5-dicyclohexyl phenol, p-phenyl phenol, p-crotyl phenol, 3,5-dimethoxy phenol, 3,4,5-trimethoxy phenol, p-ethoxy phenol, p-butoxy phenol, 3-methyl-4-methoxy phenol, and p-phenoxy phenol. Multiple ring phenols such as bisphenol A are also suitable.
The urea used to prepared urea formaldehyde resins is available in many forms. Solid urea, such as prill, and urea solutions, typically aqueous solutions are commonly available.
The melamine used in the preparation of melamine and melamine urea formaldehyde resins may be totally or partially replaced with other aminotriazine compounds. Other aminotriazine compounds include substituted melamines, cycloaliphatic guanamines or mixtures thereof. Substituted melamines include alkyl melamines and aryl melamines which can be mono-, di- or tri-substituted. Examples of alkyl substituted include monomethyl melamine, dimethyl melamine, trimethyl melamine, monoethyl melamine and 1-methyl-3-propyl-5-butyl melamine. Examples of aryl substituted melamine include monophenyl melamine and diphenyl melamine.
Aldehydes used to react with the phenol, urea, melamine and combinations thereof have the general formula RCHO wherein R is a hydrogen or hydrocarbon radical having from 1 to 8 carbon atoms. Examples of aldehydes reacted with the phenol, urea, melamine or mixtures thereof include any of the aldehydes heretofore employed in the formation of aldehyde resins such as formaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde, paraformaldehyde and benzaldehyde.
Suitable catalysts used to promote the reaction of the phenol, urea, melamine and mixtures thereof and the aldehyde are also present. Novalak type phenolic resins are typically prepared in the presence of strong acids such as sulfuric acid, sulfonic acid, oxalic acid or occasionally phosphoric acid. Novalak type resins may also be prepared using divalent metal catalysts containing Zn, Mg, Mn, Cd, Co, Pb, Cu, and Ni. Resole type phenolic resins are generally prepared in the presence of basic catalysts such as NaOH, Ca(OH)2 and Ba(OH)2. Other basic catalysts such as triethyl amine may be used to prepare resoles. The catalysts may be used alone or as mixtures. The catalysts used in urea, melamine and melamine-urea formaldehyde resins are well known in the art.
Typically, a dual catalyst system is used to prepare urea, melamine, and melamine-urea resins. Initially the reaction is carried out in the presence of a basic catalyst and completed with an acidic catalyst. Basic catalysts include any of those listed above for preparing resoles. Acid catalysts include weak acids such as formic acid acetic acid and ammonium sulfate.
According to the invention an amino acid or mixture of amino acids is added to the aldehyde resin system prior to forming the resin as a means of providing reduced free aldehyde resins. Amino acids can be obtained by hydrolysis of proteins or synthesized in various ways, especially by fermentation of glucose. Examples of suitable amino acids include lysine, L-leucene and glycine. Glycine is a preferred amino acid. It has been found that as little as 1% by weight glycine based on the total weight of resin solids can reduce the level of free formaldehyde in aldehyde resins to less than about 0.1% by weight and substantially reduce the aldehyde emissions during the curing process.
Another advantage obtained by using glycine in water based aldehyde resin systems is the increased water tolerance with time. For example, water based resole resins when stored increase in viscosity and decrease in water tolerance over time. The use of glycine in water based resole was found to increase the viscosity and also increase the water tolerance with time. In general the advantages of glycine in an aldehyde resin can be obtained by adding from 1 to 3% by weight glycine based on the total weight of resin solids. Of course amounts greater than 3% by weight of an amino acid can be used but it is generally not economically desirable
In addition to the above components, other compositions known to those skilled in the art can be added to the aldehyde resins of the present invention. For instance, stabilizers and resin modifiers, emulsifiers, plasticizers and compounds to adjust the pH can be added.
Examples of stabilizers and resin modifiers include methanol, ethanol, isopropanol, borax, and sodium sulfite. Examples of emulsifiers include casein, whey, cellulose, gum and triethyl amine. Examples of plasticizers include glycols. Compounds used to adjust the pH of the aldehyde resins include alkali metals, alkali metal hydroxides, alkali metal carbonates, alkaline earth hydroxides, organic amines, dilute mineral acids and organic acids or acid salts.
The reduced free aldehyde resins of the present invention may be used in any application that comparable aldehyde resins were used. Examples include saturants for cellulosic materials, adhesives for bonding paper, textiles, leather, metals and elastomers, in abrasives, in the manufacture of particle board, as a binder for composite panels, etc.
Having thus described the invention the following examples are illustrative in nature and should not be considered as limiting the scope of the invention. In the examples all amounts were in parts by weight unless otherwise indicated. Initial viscosities were run at 25xc2x0 C. on a Brookfield viscometer. The hot plate cure was conducted according to ASTM D 4640-86. The 121 gel time test was conducted according to ASTM 3056-96. Specific gravity was run at 25xc2x0 C. Water tolerance testing was conducted by weighing a resin sample into a vessel at 25xc2x0 C. and placing the vessel over a piece of newsprint.
Deionized or distilled water was added to the sample with stirring and the newsprint viewed by looking from the top of the vessel through the sample solution. The endpoint was reached when the newsprint could no longer be read through the solution and is expressed as weight percent water added to the sample. The pH of a solution was measured at 25xc2x0 C. on neat samples using an ACCUMET(copyright) Model 15 pH meter from Fisher Scientific. Solids were measured using a standard forced air oven technique. A known weight of a resin was placed in an oven to allow the volatiles to evaporate. After cooling, the sample is reweighed. The percent solids were calculated by dividing the final weight by the original weight, multiplied by 100. Free formaldehyde was measured according to the following procedure. Six g of sample was weighed into a flask. 45 ml of methanol was added with stirring to dissolve the sample. Bromophenol blue indicator was added to he vessel. A blank was prepared in the same manner without the sample.
Sample and blank were titrated to a blue green endpoint. If prior to titration the sample solution was blue it was titrated with sulfuric acid. If the solution was yellow it was titrated with NaOH. Subsequently, 15 ml of a 10% aqueous hydroxylamine hydrochloride solution was added to the vessel and allowed to stand for from 5 to 10 minutes. The sample and blank were then titrated with NaOH to a blue green end point.
The percent free formaldehyde was calculated by subtracting the ml of NaOH required to reach the blue green end point of the blank from the ml of NaOH required to reach the blue green endpoint of the blank from the ml of NaOH required to reach the endpoint of the sample solution, multiplying that number by the normality of the NaOH and then by 3.003 and finally dividing the result by the weight of the sample.